As the data areal density in hard disk drive (HDD) writing increases, write heads and media bits are both required to be made in smaller sizes. However, as the write head size shrinks, its writability degrades. To improve writability, new technology is being developed that assists writing to a media bit. Two main approaches currently being investigated are thermally assisted magnetic recording (TAMR) and microwave assisted magnetic recording (MAMR). The latter is described by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans. Magn., vol. 44, pp. 125-131 (2008). MAMR uses a spin torque device to generate a high frequency field that reduces the coercive field of a medium bit thereby allowing the bit to be switched with a lower main pole field.
Spin transfer (spin torque) devices are based on a spin-transfer effect that arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current passes through a magnetic multilayer in a CPP (current perpendicular to plane) configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high. Spin transfer devices are also known as spintronic devices and may have ferromagnetic (FM) layers with a perpendicular magnetic anisotropy (PMA) component where magnetization is aligned substantially perpendicular to the plane of the FM layer. These devices have an advantage over devices based on in-plane anisotropy in that they can satisfy the thermal stability requirement but also have no limit of cell aspect ratio. As a result, spintronic structures based on PMA are capable of scaling for higher packing density, which is a key challenge for future MRAM (Magnetoresistive Random Access Memory) applications and for other spintronic devices such as microwave generators. However, magnetic layers with PMA are not a necessity in MAMR applications
MAMR typically operates with the application of a bias current from the main pole (MP) across a spin torque oscillator (STO) device to a trailing shield, or vice versa, in order to generate a high frequency RF field (from an oscillation layer) while a MP field is applied from an air bearing surface (ABS) to the magnetic medium. In existing designs, spin torque is applied from only one side of the oscillation layer (OL) in the STO device. Related U.S. patent application Ser. No. 16/037,197 discloses a magnetic flux guiding device wherein a high frequency RF field is not necessarily generated. Preferably, FGL magnetization flips to an opposite direction when the applied current is sufficiently large enough. Accordingly, the write gap field flux from the MP to the trailing shield is reduced to enable a greater main pole field from the ABS to the magnetic medium. Since the required applied current for optimum FGL flipping is near the maximum value that can be tolerated to ensure good device reliability, there is a need to design an improved magnetic flux guiding device that operates with a considerable reduction in applied current. Alternatively, a magnetic flux guiding device that enables a higher degree of FGL magnetization flipping at a given current density is desirable.